1. Field
The present invention relates to the field of data security, and in particular, this invention relates to digital watermarking.
2. General Background
Each day, consumers receive information from content providers in a variety of forms such as movies, television programming and the like. When transmitted in a standard analog format, the information is somewhat protected due to the inherent nature of analog signaling. For example, a recorded movie has poorer image quality than the original analog version.
Over the last few years, there has been a growing demand for digital content. In general, "digital content" involves the transmission of one or more digitized data sets. Each "data set" includes data with perceivable content (e.g., a still image, a frame of video, alphanumeric character representations, audio, Internet Protocol "IP" commands, a program, etc.). Unlike analog, digital content can be easily manipulated without affecting the quality of the original data set. This "quality" may be measured through visual clarity of an image, audible clarity during audio playback, accuracy of characters in text or code as well as other factors. Since digital content can be easily manipulated, content providers have been hesitant in supporting digital content distribution, in part, due to the absence of a mechanism to protect against unauthorized copying and/or illegal distribution of their digital content.
Recently, digital watermarking has emerged as a technique to protect against unauthorized copying and distribution of digital content. In general, "digital watermarking" comprises an act of embedding information (referred to as a "watermark") into the data set in an unobtrusive way so that the quality of the data set is not reduced, but the watermark can be extracted as the data set is being used. This is accomplished by placing the watermark into a noise band of the data set. The "noise band" may include, for example, a few least significant bits associated with the color of each pixel of an image.
In addition, a watermark may be embedded to be resilient to various manipulations of the data set such as, for example, photocopying, scanning, resizing, cropping and color manipulation. Of course, the selected degree of resiliency is determined by the amount of information that can be embedded in a data set. As an illustrative example, if resiliency to cropping is desired, a watermark is placed in a redundant fashion in different portions of the data set. If such cropping resiliency is not desired, bandwidth consumed by such redundancy may be allocated to improve quality of the data set.
Currently, there are two types of watermarks: (i) public watermarks and (ii) private watermarks. These watermarks serve different functions. For example, a "public watermark" is readable by widely-available software and is generally used to enable a consumer of the data set to identify its source. As a result, public watermarks are used to embed copyright notices, licensing contacts or other information. This information can be obtained by consumers through use of the widely-available software. However, public watermarks are relatively simple to remove or to forge.
A "private watermark" is a digital watermark embedded using a technique similar to symmetric key cryptography, but the key is held in secrecy, known only to the person or entity applying the private watermark who is normally the original owner of the content. For reading purposes, locating the private watermark in the data set requires knowledge of the secret key, and thus, the private watermark is not easy to remove. This allows an original owner to identify copyright violations and prove ownership of the data set. However, since the secret key needs to remain confidential, reader software having access to the secret key cannot be available to the public at large. Thus, use of a private watermark, by itself, would prevent legitimate consumers from checking whether digital content is covered by foreign or domestic copyright laws, ascertaining the name of the licensing contacts, if any, who have the authority to license use of the digital content or from retrieving other information placed in the private watermark.
As a result, under the current scheme, unless both public and private watermarks are used, unauthorized copying and/or distribution of digital content will continue. Unfortunately, when applying both types of watermarks, an unacceptable amount of bandwidth is used, which affects the quality of the data set. Therefore, it would be beneficial to create a new watermarking technique which utilizes the advantages of both public watermarks and private watermarks.